The invention is related to plasma reactors in which the reactor chamber containing the plasma and the workpiece is maintained at a pressure less than atmospheric by pumping gas out of the chamber through a pumping annulus or throat. One problem is that the plasma process produces various chemical species which tend to contaminate the pumping apparatus and the surface of the pumping annulus as they are pumped out of the chamber. For example, plasma processes designed to etch silicon dioxide films, without etching silicon films or photoresist, typically employ a polymer precursor gas (such as a fluorocarbon gas) which tends to form a protective polymer film over silicon and photoresist surfaces that would otherwise be exposed to etchants in the plasma chamber. Such a film tends to not form over silicon dioxide surfaces, so that the etch selectivity to silicon dioxide is significantly improved. A problem is that the polymer accumulates as a very hard semi-permanent film on any surface exposed to the processing environment, including the pumping annulus and the pumping apparatus, unless somehow prevented from doing so.
A solution to the problem of polymer contamination is to permit the polymer film to deposit on surfaces within the primary process region over the workpiece, but block polymer precursor species from leaving the primary process region and specifically blocking the polymer precursor species from entering the pumping annulus of the reactor. This is accomplished by exploiting the fact that the polymer precursor species tend to be ionized in the plasma and may therefore be deflected by magnetic fields. Thus, plasma confinement magnets are placed around the entrance to the pumping annulus. As disclosed in U.S. Pat. No. 6,030,486, issued Feb. 29, 2000, to Loewenhardt et al., and U.S. patent application Ser. No. 09/773,409, filed Jan. 31, 2001 by Loewenhardt et al., a pair of horseshoe magnets may be placed across the entrance to the pumping annulus to deflect all ionized particles and thereby prevent them from entering the pumping annulus. The horseshoe magnets are relatively expensive. Other techniques for confining the ionized particles are also commonly used, such as baffle arrangements ahead of the pumping annulus. One such baffle arrangement entails imposing an xe2x80x9cS-curvexe2x80x9d in the wall at the entrance of the pumping annulus. The S-curve in the wall is such that there are no straight-line paths through the entrance of the pumping annulus. As a result, the particles entering the pumping annulus tend to collide with the wall of the annulus in the vicinity of the S-curve. For an appropriately proportioned S-curve, at least nearly all of the polymer precursor species collide with the wall in the vicinity of the S-curve and are deposited thereon before they can reach the interior of the pumping annulus or the pumping apparatus.
As technology advances, the workpiece size increases. For example, in microelectronic circuit fabrication, the workpiece is a silicon wafer whose diameter may reach or exceed 12 inches. This size increase leads to greater difficulty in maintaining uniform gas flow across the wafer surface, particularly at low chamber pressures. For very low chamber pressures and very large wafer diameters, the gas flow rate through the pumping annulus must be very high to maintain the low chamber pressure and uniform gas flow across the wafer surface. The larger the wafer diameter, the greater must be the gas flow rate through the pumping annulus. It has been found, for example, that for a 200 mm diameter wafer, in order to maintain a chamber pressure of 40 mT, a gas flow rate of about 100 cubic centimeters per second is required. In processing a larger wafer such as a 300 mm diameter wafer, an even lower chamber pressure may be required, such as 20 mT. In this case, a gas flow rate of about 200 cubic centimeters per second through the pumping annulus is required, and it is often difficult to maintain or even reach such a high flow rate in a conventional chamber sufficiently large to hold a 300 mm wafer.
One feature which severely limits the gas flow through the pumping annulus, and often prevents the gas flow from reaching the requisite level, are the baffle arrangements at the entrance to the pumping annulus, which has been referred to above. These necessarily induces turbulence that significantly impedes the gas flow. It appears that neither increasing the pump power nor xe2x80x9csmoothingxe2x80x9d the baffling are viable solutions to this problem. As for increasing the pump power, we have found that for a large wafer size (e.g., 300 mm diameter) and a target chamber pressure that is low (e.g., 20 mT), the impedance induced by the baffling is such that the requisite gas flow rate cannot be reached even if the pump power is increased. Also xe2x80x9csmoothingxe2x80x9d of the baffling or widening of the throat size of the pumping annulus in an attempt to increase the gas flow detracts from the ability of the baffle to block polymer precursor species, and therefore does not appear to be a practical solution to the problem.
What is needed is an inexpensive way of blocking polymer precursor species from the pumping annulus without impeding gas flow through the pumping annulus.
A plasma reactor includes a chamber-adapted to support an evacuated plasma environment, a passageway connecting the chamber to a region external of the chamber, the passageway being defined by spaced opposing passageway walls establishing a passageway,distance therebetweeen, and a plasma-confining magnet assembly adjacent the passageway. The plasma-confining magnet assembly includes a short magnet adjacent one of the passageway walls and having opposing poles spaced from one another by a distance which a fraction of the gap distance, the short magnet having a magnetic orientation along one direction transverse to the direction of the passageway, and a long magnet adjacent the other one of the opposing passageway walls and generally facing the short magnet across the passageway and having opposing poles spaced from one another along a direction transverse to the passageway by a pole displacement distance which is at least nearly as great as the gap distance, the long magnet having a magnetic orientation generally opposite to that of the short magnet.